This invention relates to the combustion of industrial waste gases having relatively low calorific value, including but not limited to the combustion of waste gases produced in carbon black plants. Recovery of heat (hence energy conservation) and/or elimination of certain atmospheric pollutants are the desired objectives of the invention.
The shortage and increasing prices of natural gas are strong incentives for development of combustion systems capable of efficiently burning such low calorific value waste gases to furnish energy which would otherwise need to be generated by consumption of natural gas or oil.
Cyclone or rotary flow combustors of the type consisting of a cylindrical combustion chamber with a restricted outlet and provided with a swirl burner mounted in the front wall of the combustor and coaxial with the combustion chamber for the introduction of waste gas and air and subsequent ignition upon entering the combustion chamber, have been successfully utilized. The aerodynamics of certain combustors of this type are characterized by rotary flow throughout the combustion chamber and a broad field of axially reverse flow in the first section occupying approximately one-third of the length of the combustion chamber. In this section hot combustion products are recirculated and mixed with the incoming mass of waste gas and air, thereby increasing the temperature sufficient to maintain them above the ignition temperature. The remaining volume of the combustion chamber, approximately two-thirds, is utilized to complete combustion of the waste gas. Numerous studies in which swirling jets are introduced in the front walls of enclosed cylindrical furnaces as a means to control and improve the combustion dynamics have been reported. The following studies describe techniques utilizing ratios between the burner or swirl chamber exit diameter and the combustion chamber diameter of one-fourth and higher, which is characteristic for the dimensions of the equipment used in the subject invention.
1. "Study of the Aerodynamics of A Furnace Space," by V. N. Afrosimova, Teploenergetika, 1967 14 (1) 9-13.
2. "An Investigation of the Behavior of Swirling Jet Flows in a Narrow Cylindrical Furnace," by H. L. Wu and N. Fricker, Chapter IX of the proceedings of the International Flame Research Foundation.
One of the objectives of this invention was to develop a burner which would be smaller and less costly than a combustor consisting of a burner assembly plus a large combustion chamber and to install a plurality of such burners in the walls of existing fireboxes used to provide the heat required in a rotary drying kiln or drum as are used for instance in, but not limited to, the drying of wet pellets in the manufacture of carbon black. In this application only the ignition section of the combustion chamber is maintained as an enclosed refractory lined cylinder, but completion of the combustion has to be achieved outside this cylinder in the firebox of the drying kiln. This application requires a temperature to sustain combustion which is higher than the temperature generated by the waste gases in the lower range of caloric value -- that is from 36 to 44 BTU/SCF Net. It is therefore essential that the burner arrangement incorporates a supporting burner for natural gas, propane or the like, or oil, to provide the heat to sustain ignition and combustion of the waste gas and even to provide sufficient heat to the drying drum to dry the desired throughput of carbon black pellets when no waste gas is available or when the heating value of waste gas has been reduced to the extent that it cannot by itself sustain combustion. Such a burner should also incorporate a pilot flame to ensure that the supporting gas or liquid fuel will ignite inside the burner assembly and not escape unburned into the firebox where it is liable to cause an explosion. The requirements for this supporting burner are: the capability to maintain combustion at high loads and at very low loads under adverse conditions; that is, with large flows of often unburnable waste gas in open contact with the flame and even to ignite under these conditions from the pilot flame. The pilot flame should also be completely unaffected by the quenching gas flow. Without this capability the entire operation of the drying drum becomes an explosion hazard. In order to maintain the desired simplicity in design and dimensional limit, it is highly desirable that this burner should either be completely enclosed within the waste gas burner assembly or extend coaxially a short distance outside this assembly.
The basic problem involved in realizing these objectives was the proper design and location of the supporting gas or oil burner, so that the supporting gas flame would not be extinguished. One or more gas burners located in the burner throat or in the upstream zone of the combustion chamber could successfully ignite the waste gas. However, in these locations, they could not survive the flow of low quality waste gas or even less than very cautiously controlled increments of good quality waste gas. These locations would also be prohibitive for a reliable pilot flame. The alternative solution would be a supporting burner upstream of and coaxial with the waste gas swirl chamber. The supporting flame should have its stable root and ignition point at the burner mouth upstream of the waste gas swirl chamber and should traverse through this swirl chamber without expanding and impinging on the unprotected swirl chamber casing. To obtain the required stability and narrow flame shape of the supporting flame within the dimensional limits again required aerodynamics based on a swirl; however, this swirl, combined with the swirl of the waste gas burner, created a central reverse flow by which waste gas penetrated into the root of the supporting flame and extinguished it. Operating the supporting burner under straight, that is nonswirling flow, would draw the flame out too long, and it would actually be extinguished when the forward part was quenched by the waste gas.